The invention relates in general to the field of liquid crystal display design and, more particularly, to the fabrication of organic compensator elements for use in a liquid crystal display. Specifically, the invention describes a birefringent optical compensator that is comprised of sequentially layered surfaces having a plurality of photo-patterned regions wherein some of the regions in a layer are birefringent while other regions in the layer are substantially isotropic.
Contrast and stability of relative grayscale intensities are important attributes in determining the quality of a liquid crystal display (LCD). A primary factor limiting the contrast achievable in current LCDs is the amount of light which leaks through the display while it is in its dark state.
In addition, the contrast ratio of a LCD also depends on the viewing angle. Contrast ratios in a typical LCD are a maximum only within a narrow viewing angle centered about normal incidence and drops off as the viewing angle is increased. This loss of contrast ratio is caused by light leaking through the black state pixel elements at large viewing angles. In color displays, such leakage is also known to cause severe color shifts for both saturated and grayscale colors.
2.1. Single Tilt Domain Liquid Crystal Display
The viewing zone of acceptable grayscale stability in a typical prior art twisted nematic liquid crystal display is severely limited because, in addition to color shifts caused by dark state leakage, the optical anisotropy of the liquid crystal molecules results in large variations in gray level transmission, i.e., a shift in the display's brightness-voltage curve, as a function of viewing angle. The variation is often severe enough that, at extreme vertical angles, some of the gray levels reverse their transmission levels. These limitations are particularly important for applications requiring a very high quality display, such as for use in avionics, where viewing of cockpit displays from both pilot and copilot seating positions is important. Such high information content displays require that the display's relative gray level transmission be as invariant as possible with respect to viewing angle. It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
As shown in FIG. 1, a conventional full color single tilt domain display 100 comprises a polarizer 105, an analyzer 110, a liquid crystal cell 115, and possibly one or more compensator layers 120. The liquid crystal cell 115 further comprises an active matrix substrate 125, a color matrix substrate 130, and liquid crystal material 135. (A polarizer 105 and an analyzer 110 both polarize electromagnetic fields. Typically, however, the term `polarizer` refers to a polarizer element that is closest to the display's source of light while the term `analyzer` refers to a polarizer element that is closest to the display's viewer.)
Ordinarily, the active matrix substrate 125 has deposited on it an array of thin-film transistors, transparent electrodes, address lines, and an alignment layer. Furthermore, the color matrix substrate 130 often has deposited on it a black matrix coating, a color filter matrix, a transparent electrode, and an alignment layer. The alignment layers on the active matrix substrate layer 125 and color matrix substrate layer 130 act in combination to induce a twisted nematic orientation to the liquid crystal material 135. Elements comprising the active matrix substrate layer 125 and color matrix substrate layer 130 are well-known and not shown in FIG. 1.
2.2. Dual Domain Liquid Crystal Display
One method of improving the grayscale linearity characteristics of a conventional liquid crystal display (LCD) is to implement a multi-domain LCD architecture. The basic structure of a dual-domain LCD is quite similar to that of the display shown in FIG. 1, with some significant differences as illustrated in FIG. 2. As shown, a collection of nematic molecules 200 are disposed between a pair of substrates 205 and 210. One substrate 205 has two rubbing directions, 215 11 and 220. The other substrate 210 also has two rubbing directions, 225 and 230. Rubbing direction 215 is opposite in sense to rubbing direction 220. Likewise, rubbing direction 225 (in a direction into the plane of FIG. 2) is opposite in sense to rubbing direction 230 (in a direction out of the plane of FIG. 2). Rubbing directions 215 and 220 are arranged at approximately 90.degree. angles to rubbing directions 225 and 230 respectively. This type of rubbing produces two tilt domains 235 and 240 within the liquid crystal layer--hence the term "dual-domain" architecture.
A conventional dual-domain LCD further comprises a polarizer 245, an analyzer 250, and possibly one or more compensator layers 255 disposed between the polarizer and analyzer. Other methods have also been reported to generate two-domain structures, e.g., Koike et al. ("Late-News Paper: A Full-Color TFT-LCD With a Domain-Divided Twisted-Nematic Structure," SID 92 Digest, pp. 798-801, 1992) and Takatori et al. ("A Complementary TN LCD with Wide-Viewing-Angle Grayscale," Proceedings 12th Int. Display Res. Conf.--Japan Display 92, pp. 591-594, 1992).
One function of a dual-domain display is to average the gray level behavior of the display over the display's positive and negative vertical viewing directions. Such averaging is known to produce improved gray level linearity.
Increasing the display's contrast over the vertical field of view requires compensation that is effective for both positive and negative viewing directions. Further, even the improved gray level linearity produced by a basic dual-domain architecture is not sufficient for many applications, e.g., avionics and large workstation displays. See, for example, Yang, "Two-Domain Twisted Nematic and Tilted Homeotropic Liquid Crystal Displays for Active Matrix Applications," Record 1991 Int. Display Res. Conf., San Diego, Calif., pp. 68-72, 1991.
2.3. O-Plate Compensation
Commonly owned, U.S. Pat. No. 5,504,603 describes a novel compensator structure incorporating an obliquely-oriented, positively birefringent compensator (phase retardation) element referred to as an O-plate compensator. The O-plate compensator structure significantly improves gray level linearity and provides high contrast over large variations in viewing direction for single-domain twisted-nematic LCD architectures. Additional embodiments allow for the use of a negatively birefringent O-plate compensator.
The optical axis (i.e., the major axis of the refractive index ellipsoid) of an O-plate compensator has a fixed orientation relative to the average orientation of the liquid crystal director near the central region of the liquid crystal cell in its partially-driven state. This orientation requirement makes use of prior O-plate compensator techniques incompatible with a dual-domain liquid crystal display architecture because the orientation of the liquid crystal director is different in the two domains.
For an O-plate compensator to be effective, the alignment of its optical axis must have a fixed relation to the liquid crystal director orientation. This requirement results in the need to "pixelate" the O-plate compensator element. That is, the orientation of the O-plate within each pixel must vary between the different tilt domains. Thus, for a dual-domain LCD employing O-plate compensation, the O-plate must be spatially patterned into two separate areas having two different orientations with the pattern coinciding with that of the alternating tilt domains.
Further, because of a display's small pixel size (typically less than 150 micrometers, .mu.m) relative to the thickness of the liquid crystal cell's substrate (typically 1.1 millimeter, mm), it is necessary to fabricate the pixelated O-plate compensator on an interior surface of the liquid crystal cell's substrate. If the pixelated O-plate were applied external to the liquid crystal cell, severe parallax problems would result. Thus, if O-plate technology is used in a dual-domain LCD, it is necessary to place at least the O-plate components and possibly other compensator components inside the liquid crystal cell.
2.4. Pixelated Compensator Architectures
More generally, a pixelated dual-domain liquid crystal display might include combinations of components such as shown in Table 1 below. Table 1 is a partial list of useful pixelated compensator configurations. In each example, the liquid crystal layer (LC) is presented as two sections having different tilt domain orientations as designated by arrows. Compensators that are internal to the liquid crystal cell and pixelated are printed on the same line as their corresponding tilt domain and designated as different valued using subscripts and by orientation using arrows. Components that are not pixelated are shown as having identical subscripts and/or orientations for both tilt domains. Here, A represents an A-plate, C represents a C-plate, O represents an O-plate, and P represents a polarizer (representing either an analyzer or polarizer). As would be apparent to those of ordinary skill having the benefit of this disclosure, pixelated components can be placed on either side of the liquid crystal layer, depending on the design parameters a designer is attempting to optimize. An A-plate is a compensator element whose optical axis is oriented substantially in the plane of the plate. A C-plate compensator element has its optical axis oriented substantially normal to the plane of the plate. As for O-plate compensator elements, A-plates and C-plates may have either positive or negative birefringence. The embodiments described on Table 1 represent configurations utilizing positively birefringent O-plates and A-plates, and negatively birefringent C-plates.
TABLE 1 __________________________________________________________________________ Pixelated Liquid Crystal Display Elements .rarw. Toward Rear (Polarizer Side) Toward Front (Analyzer Side) .fwdarw. external to LC cell - internal to LC cell - external to LC cell - __________________________________________________________________________ P LC.uparw. A.sub.1 O.sub.1 .uparw. A.sub.2 P LC.dwnarw. A.sub.1 O.sub.2 .dwnarw. P LC.uparw. C.sub.1 A.sub.1 O.sub.1 .uparw. A.sub.2 P LC.dwnarw. C.sub.1 A.sub.1 O.sub.2 .dwnarw. P C.sub.1 LC.uparw. C.sub.2 A.sub.1 O.sub.1 .uparw. A.sub.2 P LC.dwnarw. C.sub.2 A.sub.1 O.sub.2 .dwnarw. P A.sub.1 LC.uparw. A.sub.2 O.sub.1 .uparw. P LC.dwnarw. A.sub.2 O.sub.2 .dwnarw. P C.sub.1 LC.uparw. A.sub.1 O.sub.1 .uparw. P LC.dwnarw. A.sub.1 O.sub.2 .dwnarw. P C.sub.1 LC.uparw. A.sub.1 P LC.dwnarw. C.sub.2 A.sub.2 P LC.uparw. A.sub.1 O.sub.1 .uparw. A.sub.3 P LC.dwnarw. A.sub.2 O.sub.2 .dwnarw. P C.sub.1 LC.uparw. P LC.dwnarw. C.sub.2 __________________________________________________________________________